The brooding bivalves, Nutricola tantilla and N. confusa overlap in their geographic distributions,
habitats, and modes and timing of reproduction. Based on previous studies we infer that males of both species release
spermatozoa into the water column; while females retain developing embryos in a brood chamber. Females release
fully formed juveniles and there is no pelagic larval stage. We hypothesized that the muco-ciliary processes of particle
selection and retention may act on differences in sperm morphology and contribute to reproductive isolation. We
extracted sperm cells from both species and quantified nine linear measurements: the lengths of the acrosome,
nuclear, midpiece and tail regions, and five different width measurements. We found significant differences in the
lengths of the acrosome, midpiece, and tail. We also found that N. confusa produces dimorphic sperm and this is the
first report of sperm dimorphism in the Veneroidea. Despite the significant differences in lengths, it is likely that
other prezygotic mechanisms are responsible for reproductive isolation.